Since the invention of the electric motor, continuous progress has been made in centuries and results in huge contributions to the welfare of people's lives. Initially, the electric motors were mainly built in larger sizes for industrial purposes. It has since evolved to a smaller size and applied to home appliances and machine tools. With the advance of fabrication materials and technologies, the size of the electric motor has become smaller while applicability has increased significantly.
Nowadays a wide variety of electric motors are available according to structural types. Each type of motor has various application characteristics in terms of size, heat dissipation, torque, rotation speed, material, production cost, control method and the like. As each electric motor is different, it is difficult to assert which one is best. Selecting the optimal one has to take use environments and requirements in account. On the smaller electric motor the most challenging use condition and environment is the one for the machine tools.
The electric motor used on the machine tools has to meet a great demand on all of its characteristics, such as compact size, sufficient torque, high rotation speed and greater durability used for a prolonged period of time. The deficiency of any characteristic mentioned above could cause a dysfunction or damage of the machine tool. Another concern needing to be addressed is accumulated heat generated by the bearing, coil of motor stator and electromagnetism. If not properly done, it could cause the motor to overheat and directly impact operation duration of the machine tool, namely, affect the durability of the machine tool.
The electric motor generally used on the machine tools is a brushed DC motor with a brush and a commutator. It usually is bulky and has a lower efficiency. Sparks are generated between the brush and commutator which results in the wearing of these two elements. They become consumption items and have to be replaced regularly. Therefore the motor for machine tools gradually is displaced by the brushless DC motor of a smaller size and higher efficiency. Its smaller size results in the concentration of heat generated by the motor. The heat tends to accumulate in a sealed housing when it is used, as the brushless DC motor is encased in a machine tool and is difficult to discharge. As a result, overheating could occur to influence machine tool operation. To overcome this problem many heat dissipation structures have been developed. References can be found in U.S. Pat. Nos. 6,789,630 and 7,166,939, U.S. publication No. 2008/0233848, and R.O.C. patent No. M263204. The most commonly used brushless DC motor on machine tools is an inner runner motor. It mainly has a shaft to drive a fan or heat sink attached thereon. When the main motor rotates the fan or the heat sink is driven to rotate simultaneously, and an airflow is generated to perform heat dissipation. There is another technique with a cooling motor equipped with a fan. Whether the main motor rotates or not, the cooling motor continuously operates to perform cooling. However, such an approach has a limited effect when the machine tool is in use for a long duration. The size and elements needed also increase, and fabrication costs and electric power consumption also are higher. The brushless DC motor used on the machine tools generally does not adopt the out runner motor. It also has a cooling problem identical to the inner runner motor. There are also prior techniques about the cooling structure of the out runner motor, such as R.O.C. patent No. M269645. It mainly has an out runner with an air inlet formed thereon. The housing has a corresponding aperture. Airflow is sucked in through the rotation of the out runner and discharged through the aperture to achieve cooling effect. However, the cooling structure of the brushless DC motor with the out runner disturbs airflow merely through an inclined tangent surface on the air inlet of the out runner. It has a limited effect in discharging the accumulated heat in a prolonged use condition. Hence there is still a lot of room for improvement on the brushless DC motor used on the machine tools to solve the problem caused by poor heat dissipation.